This invention is concerned with a method of and apparatus for measuring gas pressures over a wide range from high vacuums to pressures greater than atmospheric. The primary application of the invention is in the measurement of vacuums.
Known vacuum gauges can be divided into those which measure the pressure directly and those which measure pressure-dependent gas parameters. The advantage of vacuum meters of the first-mentioned type resides in the fact that they are independent of the type of gas if the gas laws for a perfect gas are substantially followed. Among the direct pressure measuring gauges there are those in which a diaphragm or membrane is deformed or deflected by the gas pressure, that deformation or deflection being measured and those wherein the gas pressure pushes a column of liquid upward against the force of gravity, the shift in height of the column being measured.
The Pirani vacuum meter, which utilizes the pressure-dependent heat conductivity of gases for measuring pressure is typical of the second type of gauge as is the Penning vacuum meter of which pressure measurement is based on the pressure-dependency of the current intensity of an automatic gas discharge. Also of the second type of gauge is the friction vacuum meter which utilizes the pressure-dependency of the friction coefficient of the gas and the ionization vacuum meter where the pressure dependency of the ionization degree of the gas caused by electron impact, is used for measuring.
Membrane vacuum meters require a complicated production technology and are, therefore, expensive; further reproduction of the zero point or calibration, is difficult. Box vacuum meters are not suited for exact measurements and liquid vacuum meters entail the shortcoming that vaporization of the measuring liquid occurs in the measuring vessel and the measuring range is very small during a continuous measurement.
Vacuum gauges wherein the pressure dependency of a gas parameter is utilized for measuring the pressure have the disadvantages of being dependent upon the type of gas of which the pressure is to be measured and of the non-linearity of the relationship of the parameter to be measured with the pressure. The Penning vacuum gauge has the additional shortcoming of relatively great measurement inaccuracies; the gas to be measured can disintegrate in the ionization vacuum meter and the Pirani vacuum meter has only a small measuring range.
The present invention seeks to provide a method for continuously measuring gas pressures and apparatus for use in that method which permit direct measuring independently of the type of gas. No liquids are to be used in accordance with the invention and the measuring process should not alter the gas composition. The absolute measuring exactness should be better than 2% where the gas laws for a perfect gas are followed to within 2%. The device should measure linearly so that its calibration can be completely effective at a single pressure point and the same measuring scale be used for all measuring ranges.